However, I just got a fat bike with 4.5in tires. I know that my portable hand pump takes a long time to pump mountain bike tires due to it being optimized for high pressure / low volume. What type of portable pump should I carry in my toolkit when riding on the fat bike for emergency repairs and adjusting air pressure out on the trails away from the car or home?
I have/do carry an Lezyne Allow Drive pump. I have spent a lot of time pumping fat tires in really cold temperatures and although it's not as quick, it's compact and packs well. That Lezyne floor version doesn't look too bad, but it is still a bit big to pack for my tastes. It looks like they have a specific HV hand pump out that is designed like their others.
I would recommend against ANY pump with ANY plastic parts if you ever intend to ride in cold weather. I have seen far to many pumps (like the Mountain Morph) that have plastic parts get brittle in the cold and snap. Being able to pump up a tire in two minutes less doesn't mean anything if the pump breaks. I would only ever recommend pumps for cold weather that have a non-plastic screw on attachment and no plastic structural parts.